Amino acids are very important components in industries as, e.g., drugs, food, reagents, and chemical synthesis intermediates. Methods for producing an amino acid are roughly divided into four methods, i.e., an extraction method, a fermentation method, an enzyme method, and a chemical synthesis method. The enzyme method is a method in which a precursor having a similar structure to that of an objective amino acid as a starting material is converted at once into the amino acid through one to several stages of enzyme reaction. Generally, the enzyme method gives an amino acid having high purity with little amount of by-products. When the precursor serving as the substrate is available inexpensively, the enzyme method is a very efficient production method.
Monatin, one kind of amino acid derivative, is a natural occurring, sweet-tasting amino acid isolated and extracted from roots of shrubs in South Africa. Monatin has sweetness that is several ten times to thousand times stronger than that of sucrose, and is expected to be used as a sweetener.
As an example of a chemical synthesis method for producing monatin, there is a method wherein an indolacetic acid derivative and an aspartic acid halide are used as starting materials to synthesize a ketone derivative, and a cyanohydrin derivative is obtained therefrom, which is then hydrolyzed under basic conditions (for example, Patent Document 1). As an example of an enzyme method, there is a method wherein 4-(indol-3-ylmethyl)-4-hydroxy-2-oxoglutarate (also called IHOG) is formed as an intermediate from indol-3-pyruvic acid, and then monatin is produced therefrom in the presence of an enzyme (for example, Patent Document 2). There is also known a method for producing the aforementioned IHOG in the presence of an enzyme (for example, Patent Document 3).
As another method for forming monatin from IHOG, there is a method wherein IHOG is used to produce IHOG-oxime (or 4-hydroxy-4-(3-indolylmethyl)-2-hydroxyiminoglutarate) which is then converted into monatin in the presence of a reducing catalyst such as rhodium (for example, Patent Document 4). However, there is not known any method for producing monatin from IHOG-oxime which is more stable than IHOG, in the presence of a microorganism or an enzyme.
Reduction of oxime (hydroxyimine) by a microorganism or an enzyme (enzymatic reduction) is described in Patent Document 5 and Nonpatent Documents 1 to 4. For example, the Patent Document 5 describes a method for producing α-methylbenzylamine from acetophenone oxime. However, there is not known any method for reducing hydroxyimino acid in the presence of a microorganism or an enzyme to produce an amino acid.